1. Field of the Invention
The present invention relates generally to color cathode ray tube (CRT) display assemblies of the well known shadow mask type and more particularly to an improved dynamic beam convergence control apparatus for improving the color resolution of the displays generated thereon. Although not so limited, the present invention is particularly useful in color CRT flight instruments for aircraft for displaying control and navigation information to the pilot where the eye screen distance is relatively short, i.e., on the order of 26 to 30 inches. This information may be presented in raster format and/or stroke written format, the latter particularly being degraded if superior dynamic convergence control, such as provided by the present invention is not employed.
2. Description of the Prior Art
The problem of convergence control in shadow mask type color CRT displays has in the past been solved to an acceptable degree by many methods and techniques. As is well known to those skilled in the shadow mask CRT art, three separate very thin beams of electrons are generated, usually from three separate cathodes, which beams are normally focused on a screen or mask spaced from the interior surface of this CRT faceplate, which screen is comprised of myriads of pin-holes through which the beam triad passes and then diverges to energize corresponding dot triads of red, green, and blue phosphors resulting in red, green and blue light emissions from the face of the CRT. It is well understood that in the assembly of a shadow mask CRT, the faceplate and mask are a unique set; the black matrix and each phosphor dot is precisely located on the screen using a direct etch process. The phosphors contain photoresist material which is exposed through the mask by a light source which is directed by a suitable correction lens to pass through each mask hole in the same direction as its respective electron beam will pass in normal operation. Thus, pure colors are assured if the electrons in the beams traverse the same path as the photons of the lighthouse direct each beam. In a conventional delta gun configuration, the three electron guns, usually in an integrated assembly located at the end of the tube neck, are tilted towards each other at a precise angle such that their beams will converge at the center hole of the mask. However, it will be appreciated that manufacturing tolerances in the guns, the gun mounts, tube neck, and mask and faceplate geometrics will contribute to a non-convergence of the beams, i.e., all beams will all not pass through the same hole at all deflection angles. Furthermore, inherent non-uniformity in or non-linearity of the magnetic field generated by the beam deflection coils and the slightly diverse paths of each beam through those fields will also tend to produce misconvergence problems. The latter may be reduced at least to some extent by tailoring the deflection yoke coils to its particular CRT and then potting the two together. Nevertheless, in practice perfect or near perfect convergence may not be accomplished solely by mechanical means or deflection coil design. For example, misconvergence will inherently occur due to changing beam lengths as the beams are deflected towards the peripheral areas of the faceplate; the same familiar pincushion phenomenon. While this effect, which is approximately proportional to the square of the distance from the screen center, can be in major part electronically compensated by providing separate external beam convergence coils associated with gun pole pieces, such compensation is not usually completely uniform and varies from tube to tube so that the desired convergence or color resolution may not be achieved by this means only, further misconvergence compensation is required.
Some prior art schemes for minimizing the misconvergence problem have been developed, one of the most effective being arranging the electron guns in a linear array and shaping the deflection coil to provide inherent convergence control. While this technique has proved satisfactory for the commercial TV market, it is not appropriate for special applications such as aircraft cockpit instrumentation or other close-up observation CRTs because it suffers primarily from a small focus lens and incompatibility with larger neck CRTs which are desirable from a high performance display standpoint. Thus, a delta gun arrangement is desirable in that it provides a larger focus lens and therefore smaller, cleaner, brighter lines and a better overall aesthetic display, although dynamic convergence control is required. One prior art convergence compensation technique in delta gun CRT configurations involved, in addition to the basic parabolic x.sup.2, y.sup.2 compensation voltages applied to each of the three convergence coils representing the primary terms of the X and Y convergence compensation series exponential polynomials, the separate adjustment of a large plurality of potentiometers to supply the required fine compensation voltages to the coils adjacent to the gun triad. However, the potentiometers were used simply as curve shapers or gain changers for the x.sup.2, y.sup.2 signals for distorting the x.sup.2, y.sup.2 parabolic curve to simulate the effects of the minor polynomial terms. One potentiometer was required in each segment of the screen so that compensation was limited by space and weight considerations. Therefore such a technique is highly undesirable in airborne CRT displays because of the limited compensation provided and the resultant large volume, weight, reliability and cost penalties involved, together with cumbersome calibration procedures typically associated with interacting potentiometers.